Simultaneous NH/NO PLIF measurements in plasma-assisted ammonia and ammonia/hydrogen swirling flames

This study investigates simultaneous NH/NO planar laser-induced fluorescence (PLIF) measurements in plasma-assisted NH₃/air and NH₃/H₂ (9:1 volume ratio)/air flames at equivalence ratios of $\varphi =0.75$, $\varphi =0.94$, and 1.1. A single dye laser system, equipped with frequency-doubling and mixing units, was employed to simultaneously generate excitation wavelengths near ${\lambda }_{\text{NO}}=236.214$  nm and ${\lambda }_{\text{NH}}=303.545$  nm, enabling optimized excitation of NO and NH fluorescence, respectively. Across all equivalence ratios, plasma was found to enhance NO and NH concentrations in both NH₃/air and NH₃/H₂/air flames in the near field, although NH enhancement was less pronounced in the NH₃/H₂/air cases. In NH₃/air flames, NO concentrations decreased faster downstream with plasma activation, whereas in NH₃/H₂/air flames, NO levels remained relatively unchanged regardless of plasma activation. For NH₃/air flames, plasma could enhance atomic O production therefore acceleration of NH₃/NH₂/NH and form OH at the same time. The enhanced OH levels further promote NH production via NH₂ $+$ OH $\to$ NH $+$ H₂O in NH₃/air flames, though this effect is less significant in NH₃/H₂/air flames owing to the contribution of H₂ on radical pool buildup and less NH₃ availability in NH₃/H₂/air mixtures. In the downstream region, the reaction NH $+$ NO $\to$ N₂H₂ $+$ H plays a key role in reducing NO emissions in NH₃/air flames with plasma activation. These findings provide new insights into plasma-enhanced NH₃ flame chemistry and pollutant formation pathways, contributing to the development of cleaner and more efficient NH₃-based combustion technologies.